The expression of intracisternal A-particles (which occur in large numbers) and a nucleoplasmic protein (termed germinal vesicle antigen) during specific stages of early mouse development is well documented. We propose to study the biochemical parameters of the expression of these components with respect to both the protein products and the specific RNA sequences involved. The amounts of A-particle structural protein and germinal vesicle antigen in embryos at different stages of development will be measured by radioimmunoassay using specific antisera to related proteins of A-particles from tumor cells and murine leukemia virus, respectively. Rates of synthesis and turnover and biochemical properties of these proteins in the embryos will also be studied. The A-particle associated RNA and the mRNA for the germinal vesicle antigen will be detected and assayed by hybridization with labeled cDNA probes produced to the related viral (particle) genomes. It is expected that these studies will contribute to our understanding of the control of genetic expression in the early embryos. These studies should also better define the relatedness of the embryo A-particles to those found in tumor cells and the degree of homology that exists between the germinal vesicle antigen and the p30 protein of some strains of murine leukemia virus.